# Decoherence and Visibility Enhancement in Multi-Path Interference

## TL;DR

This paper explains how, in multi-path quantum interference, environmental decoherence can unexpectedly increase fringe contrast under certain conditions, challenging traditional measures of wave coherence.

## Contribution

It introduces a model for multi-path interference with decoherence, showing conditions where decoherence enhances fringe visibility and proposing quantum coherence as a better quantifier.

## Key findings

- Decoherence can increase fringe contrast in multi-path interference.
- Traditional visibility may fail to quantify wave nature in multi-path setups.
- Quantum coherence based on the l1 norm remains a reliable measure.

## Abstract

We theoretically analyze the observations reported in a four-path quantum interference experiment via multiple beam Ramsey interference [Phys. Rev. Lett. 86, 559 (2001)]. In this experiment, a selective scattering of photons from just one interfering path causes decoherence. However, contrary to expectations, there is an increase in the contrast of the interference pattern, demonstrating that path selective decoherence can not only lead to a decrease, but under certain conditions, to an increase of the fringe contrast. Here we explain this seemingly counter-intuitive effect based on a model for a multi-path interference, with four to six slits, in the presence of decoherence. The effect of the environment is modeled via a coupling to a bath of harmonic oscillators. When decoherence is introduced in one of the multiple paths, an enhancement in fringe contrast is seen under certain conditions. A similar effect is shown to appear if instead of path-selective decoherence, a selective path detector is introduced. Our analysis points to the fact that while traditional fringe visibility captures the wave nature in the two-path case, it can fail in multi-path situations. We explain the enhancement of fringe visibility and also show that quantum coherence based on the l1 norm of coherence, in contrast to traditional visibility, remains a good quantifier of wave nature, even in such situations. The enhancement of fringe contrast in the presence of environmental decoherence underscores the limitations of traditional visibility as a good measure for wave nature in quantifying complementarity and also makes it an unlikely candidate for quantifying decoherence. Our analysis could lead to better insight in ways to quantify decoherence in multi-path interference, and in studies that seek to exploit quantum superpositions and quantum coherence for quantum information applications.